Wrap a strip of paper or string around the base of the finger
Mark where the ends meet
Measure the length in millimeters with a ruler
Compare the measurement to a ring size chart
Repeat the measurement 2 to 3 times for accuracy
Measure at the end of the day when fingers are slightly larger
Make sure the paper or string is snug, not tight
Measure the finger you will wear the ring on
Check the knuckle size if the ring must slide over it
Use an existing ring that fits well and measure its inner diameter
Compare the inner diameter to a ring size chart
Use a printable ring sizer if available
Confirm the size using both circumference and diameter if possible